Aluminum powder, bronze powder and pearl pigments have so far been used as pigments for obtaining drawn lines of a metallic color tone having metallic luster such as gold and silver.
Since bronze powder has a large specific gravity and is susceptible to oxidative discoloration, the addition of the powder having a particle diameter of about several .mu.m which is required for providing the drawn lines with a metallic lustrous tone is liable to cause problems such as separation by settling, discoloration of the color tone and clogging at the pen tip, and it is very difficult to use the bronze powder for a water based ink for writing instruments.
Aluminum powder has a smaller specific gravity and a less discoloring property as compared with those of bronze powder. However, it has a larger specific gravity as compared with those of conventional pigments and has a defect of oxidative discoloration in a high pH area or a low pH area and a problem that the lines drawn on writing paper having a dark color develops only a silver color.
A pearl pigment comprises mica coated on the surface thereof with titanium oxide and is characterized by that it can provide a metallic lustrous property and a tint of a rainbow color depending on a difference in a refractive index originating in the thickness of the titanium oxide layer.
However, in order to obtain metallic luster close to that of metal powder, a particle diameter of 30.mu. or more is required, and the addition thereof to an ink has caused problems such as separation by settling, clogging at the pen tip and an increase in writing-resistance.
Inks for marking pens using aluminum powder as a metal powder pigment include inks described in Japanese Patent Publication No. Sho 62-37678 and Japanese Patent Publication No. Hei 1-56109. Disclosed in Japanese Patent Publication No. Sho 62-37678 is a double coloring ink composition which comprises a metal powder pigment such as aluminum powder, an oil-soluble dye, a resin and a solvent and in which the dye is penetrated and diffused into the circumference of the drawn lines formed from the metal powder pigment to produce an outline effect.
Disclosed in Japanese Patent Publication No. Hei 1-56109 is a metallic lustrous ink for a marking pen which comprises fine metal powder such as aluminum powder subjected to surface treatment, a resin and a solvent and which has a good ink follow-up property and does not produce a hard cake by the settling of the metal powder pigment in the ink reservoir of the pen barrel.
The ink for a marking pen described above has a problem that since it has a low viscosity and can not inhibit the aluminum powder from settling and separating, a stirring member such as a metal ball is put in the ink reservoir together with the ink, and this marking pen has to be shaken before writing to disperse once again the settled metal powder pigment, otherwise the good ink follow-up property can not be obtained.
Proposed as well are inks for a ballpoint pen which do not use a stirring member such as a metal ball unlike the ink for a marking pen described above.
Disclosed in Japanese Patent Application Laid-Open No. Sho 60-186573 is an ink which is characterized by containing a solvent, a thickening resin soluble in this solvent, a metal powder pigment and a color pigment at least in each prescribed amount and having a viscosity of a prescribed value or more and which has a metallic color and less precipitation of the pigment because of a high viscosity and is suitable for use in a pressurized ballpoint pen.
Further, disclosed in Japanese Patent Application Laid-Open No. Hei 1-210478 is a water based metallic lustrous ink which is characterized by adding an acetyl alcohol derivative as an additive to a primary ink comprising a resin, an aluminum paste and water and which prevents stain caused by pin hole in applying.
These conventional techniques have problems on separation by settling, an ink follow-up property and long term stability for water based inks for ballpoint pens capable of writing under an atmospheric pressure.
Provided in Japanese Patent Application Laid-Open No. Hei 7-118592 is a water based metallic lustrous ink for a ballpoint pen which is provided with a viscosity of 10,000 to 15,000 cps by a pearl pigment and a thickener comprising seed polysaccharides. However, it has such problems that the ink follow-up property is reduced because of the high viscosity and the writing-resistance is increased since the particle diameter required for providing the metallic luster is too large. Accordingly, it is not satisfactory.
Provided in Japanese Patent Application Laid-Open No. Hei 7-145339 is an ink which comprises aluminum powder, natural polysaccharides for thickening, a water-soluble organic solvent and water and in which the viscosity is 100 to 1,500 poise and a ratio of the viscosity at 1 rpm to the viscosity at 10 rpm is 3.0 or more. However, a problem on the inferior ink follow-up property because of the high viscosity still remains unsolved, and since a water-soluble dye is used for complementing a gold color, a dyeing property of the dye is not sufficient when written on writing paper having a dark color, so that only the silver color drawn lines can be obtained. Further, the lines drawn on writing paper having a pale color have a problem that they are inferior in water resistance and weatherability, so that the drawn lines are discolored. Accordingly, it is not satisfactory.
Provided in Japanese Patent Application Laid-Open No. Hei 6-313142 is a water based metallic lustrous ink comprising aluminum powder, a water-soluble dye, a resin and an inorganic salt, but it has problems on a coloring property on writing paper having a dark color, discoloration of the aluminum powder by oxidation and production of hydrogen and therefore is not satisfactory as well.
A subject of the present invention is to provide a water based ink having a metallic lustrous color for a ballpoint pen which can provide drawn lines having various metallic colors of a rainbow color tone and has improved water resistance and weatherability and a good follow-up property and which can write continuously and is capable of being stored over a long period of time.